Brain cell activity can be boosted by having a healthy heart, according to scientists. Photograph: Getty Images

As all those of middle age who have ever fumbled for a name to fit a face will believe, the brain begins to lose sharpness of memory and powers of reasoning and understanding not from 60 as previously thought, but from as early as 45, scientists say.

Their evidence comes from a large study of more than 7,000 civil servants aged between 45 and 70. The 5,000 men and 2,000 women agreed to undergo verbal and written tests on three occasions over a 10-year period for what is called the Whitehall II study.

A deterioration in the memory and thinking powers of the oldest volunteers might be expected, but in fact the researchers, led by Archana Singh-Manoux from the Centre for Research in Epidemiology and Population Health in France and University College London in the UK, found that the brains of even the youngest were already on the slide. Over the decade, there was a 3.6% decline in the mental reasoning of men and of women aged 45 to 49. The process appeared to have speeded up in the older age groups. Men aged 65 to 70 have a decline of 9.6% while women fared a little better, at 7.4%.

It matters, say the scientists, because those whose brains appear to deteriorate fastest may be more likely to develop dementia in later life – and because if there is any chance of slowing that process, those at highest risk may need to be detected and treated at an early stage, before Alzheimer's or another form of dementia becomes apparent.

Singh-Manoux and colleagues say that "understanding cognitive ageing will be one of the challenges of this century", as the number of elderly people around the globe rises exponentially.

This will have a profound impact both on individuals and on the societies they live in, they say.

The researchers used a range of tests to measure the mental performance of the civil servants. One of these, called Alice Heim 4-I, is a series of 65 verbal and mathematical reasoning problems which get progressively more difficult. They test "inductive reasoning, measuring the ability to identify patterns and infer principles and rules", say the researchers in their paper in the British Medical Journal. Participants had 10 minutes to get through them.

Then there was a test of short-term verbal memory. They were given a list of 20 words of one or two syllables and asked to write down all they could remember in any order within the next two minutes. They were tested on verbal fluency – asked to write down as many words beginning with the letter S within one minute and then as many animal names as they could, also in 60 seconds. Finally there was a multiple choice vocabulary test of the meaning of 35 words.

There are many very well-educated people in the civil service, but each individual's educational background was taken into account so that it did not influence the results.

The scientists' conclusion is that deterioration of the brain sets in earlier than most of us would have hoped, but there are things that can be done about it.

Looking after the heart, they say, has been shown to help the head. In fact, people with high blood pressure, obesity and high cholesterol who are at high risk of heart problems, are also at higher risk of dementia, studies show.

"There is enough evidence to show the importance of healthy lifestyles and cardiovascular risk factors in adulthood for dementia," they write in the BMJ. "For some of these risk factors, such as obesity, hypertension, and hypercholesterolaemia, it is mid-life levels that seem to be more important than those measured at older ages."

That message was echoed by Professor Lindsey Davies, president of the Faculty of Public Health. "This study provides extra encouragement to young and middle aged people to take care of their health – and not just for a few weeks in January.

"The risks to mental health and wellbeing can persuade people that it is unwise to only take action if and when their bodies and minds break down. There will be no single pill to help us do this: it's about the common sense approach of keeping fit and watching what we eat and drink," she said.

"Further research in this area would help public health experts be even more effective in taking preventative action to protect and improve older people's health. We need only look at the problems that childhood obesity rates will cause if they are not addressed to see how important it is that we take 'cradle to grave' approach to public health."

• This article was amended on 6 January 2012 to add the name of the study, called Whitehall II.